Generally, epoxy resins and cured produces therefrom are widely used in applications as electric materials, paints, adhesives, civil engineering and construction materials and composite materials, and depending upon the particular use, various characteristics such as heat resistance, toughness, flexibility and chemical resistance are required.
Commercially available epoxy resins such as a glycidyl ether of Bisphenol A and a glycidyl ether of Bisphenol F are normally liquid. They are admixed with curing agents and optionally fillers, and are cured to various produces for various applications. Such cured products of diglycidyl ethers of Bisphenols A and F are, however unsatisfactory in flexural strength and in flexural modulus, and do not have sufficient roughness which is required in many applications.
Accordingly, in many applications where toughness is required, cured products of epoxy resins having a high epoxy equivalent such as those represented by phenoxy resins have heretofore been used. However, epoxy resins having high epoxy equivalent such as those represented by the phenoxy resins are normally solid, and thus, upon preparation of cured products it is necessary to dissolve the solid resins in an appropriate solvent. This is inconvenient when compared with normally liquid epoxy resins such as diglycidyl ethers of Bisphenols A and F with which curing agents and fillers may be directly admixed.
For the production of epoxy foams known in the art is a process in which a system comprising an epoxy resin and a curing agent, having a foaming agent (e.g. gases such as air and low boiling compounds such as pentane) introduced thereto is reacted and foamed at room temperature. Also known is a process in which a system comprising an epoxy resin and a curing agent, having a foaming agent (e.g. azobisisobutyronitrile and azodicarbonamide) added thereto is heated and foamed at an elevated temperature.
The epoxy foams produced by such known processes have, however, problems in that they are not uniform in their foamed structure resulting in brittleness, poor mechanical durability, and have some limitations in application. They do not work well for anticorrosive agents of metals, heat insulators and shock absorbers.
It is desired therefore to provide epoxy resin foams which are uniformly foamed, and excellent in heat resistance, in adhesiveness as well as in mechanical properties.